fix: mark the open chat with aria-current in the sidebar (WCAG 1.4.1, 4.1.2) (#27502)

On latest `dev`, the chat that is currently open is indicated **only** by a background tint: `bg-black/[0.035]` in light mode and `dark:bg-white/[0.045]` in dark.

Against the page background that is **1.07:1** in light and **1.05:1** in dark. It is close to imperceptible for sighted users, and it carries no programmatic state at all, so assistive technology has no way to tell which entry in the list is the one being viewed.

Breaks WCAG 1.4.1 Use of Color (Level A), since the state is conveyed by colour alone, and 4.1.2 Name, Role, Value (Level A), since the state is not exposed.

Fix: set `aria-current="page"` on the chat link when it is the open chat, using the same `id === $chatId` condition that already drives the visual highlight, so the two cannot drift apart. `'page'` is the correct token because the trigger is a real navigation to `/c/{id}`.

This matches the existing pattern in `routes/(app)/workspace/+layout.svelte` and `chat/Placeholder/ChatList.svelte`, which already set `aria-current` for their active entries.

Verified that the bits-ui `LinkPreview.Trigger` forwards unknown attributes to the rendered anchor and does not set `aria-current` itself, so the attribute reaches the DOM.

This does not change the visual contrast of the highlight, which is worth addressing separately.

Severity: Serious. In a long chat list there is no reliable way to tell which chat is open.

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src/lib/components/layout/Sidebar/ChatItem.svelte

@@ -558,6 +558,7 @@
 							: 'bg-black/[0.035] dark:bg-white/[0.045] selected'
 						: ' hover:bg-gray-50 dark:hover:bg-gray-900 group-hover:bg-gray-50 dark:group-hover:bg-gray-900'}  whitespace-nowrap text-ellipsis transition"
 				href="/c/{id}"
+				aria-current={id === $chatId ? 'page' : undefined}
 				onclick={() => {
 					openPreview = false;
 					dispatch('select');